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Cite this: Chem. Commun., 2014, 50, 8915
A fluorescent metal–organic framework for highly selective detection of nitro explosives in the aqueous phase†
Received 24th April 2014, Accepted 16th May 2014
Sanjog S. Nagarkar, Aamod V. Desai and Sujit K. Ghosh*
DOI: 10.1039/c4cc03053b www.rsc.org/chemcomm
Selective and sensitive nitro explosive detection by a porous luminescent metal–organic framework has been reported. For the first time MOF based selective explosive detection in the presence of other nitro analytes in aqueous media is demonstrated.
Selective and sensitive detection of highly explosive and explosivelike substances has become a serious issue concerning national security and environmental protection.1 Nitro explosives such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), 2,4,6trinitrophenol (TNP), and nitrobenzene (NB) are the main constituents of numerous unexploded land mines used during World War II, and even in land mines today.1c Amongst these nitro explosives TNP has higher explosive power than TNT and is commonly used in dyes, fireworks, matches, glass, and leather industries.2 In addition to its explosive nature, TNP has also been recognized as a toxic pollutant. TNP and its mammalian metabolite, picramic acid, are known for their mutagenic properties.3 TNP is released into the environment during commercial production and use, leading to the contamination of soil and aquatic systems. Thus the selective and sensitive detection of TNP present in soil and ground water is very important for tracing buried explosives and environmental monitoring near industrial areas. However, selective and sensitive detection of TNP in the presence of other nitro compounds in water is a challenge due to their strong electron affinities, leading to false responses.4 Although current explosive detection methods, including trained canines and modern analytical techniques, are selective and accurate they suffer from disadvantages like high operational cost and portability issues during in-field use.5 Fluorescence based detection methods have attracted great attention recently by virtue of their high sensitivities, portability, short response times, and applicability in both the solid and solution Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pashan, Pune, India 411008. E-mail: [email protected]; Fax: +91 20 2589 8022; Tel: +91 20 2590 8076 † Electronic supplementary information (ESI) available: For details of MOF synthesis, PXRD patterns and photo physical studies of the MOF. See DOI: 10.1039/c4cc03053b
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phase.6 A variety of materials including conjugated organic molecules, nanoparticles and metal complexes have been employed for fluorescence based explosive detection.7 Despite this fact, their widespread use is limited due to stability, toxicity, multi-step processing and lack of control over molecular organization.8 Metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) have been extensively used for gas storage/separation, catalysis, sensing, optoelectronics, clean energy and biomedical applications owing to their high surface areas, designable architectures and host–guest interactions.9 In particular, as luminescent sensors MOFs provide several advantages over conventional fluorophores.10a,b Their designable architectures allow improved host– guest interactions and act as pre-concentrators for target analytes. Additionally, the immobilization of organic struts in MOFs gives rise to strong emissions due to reduced non-radiative relaxation. The infinite number of choices of organic linkers and/or metal centers allow the fine tuning of the electronic properties of MOFs. Furthermore, the introduction of secondary functional groups endorses preferred binding of the chosen analyte giving rise to better selectivity. For in-field selective detection of nitro explosives present in soil and ground water, probe working in aqueous media is highly desirable. Although MOFs have been employed for liquid phase explosive detection, to the best of our knowledge there are no reports of MOF based probes which can selectively detect nitro explosives in aqueous media.10 Recently, we reported the fluorescent MOF [Cd(NDC)0.5(PCA)]Gx, which exhibits a highly selective response towards TNP even in the presence of competing nitro analytes due to free basic sites.11 However, like most MOFs its poor water stability limits its application to organic solvents. The pore size, which is smaller than the analytes, prevents the pre-concentration of analytes in the coordination space of the MOF. Moreover, the toxic nature of Cd(II) hinders its widespread use in environmental applications. In our efforts to develop fluorescent MOFs that work in aqueous media for the selective detection of nitro explosives, we became interested in 2-phenylpyridine-5,4 0 -dicarboxylic acid (LH2) and the Zr(IV)
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Scheme 1 A fluorescent MOF based sensor for highly selective nitro explosive detection in the aqueous phase.
based MOF Zr6O4(OH)4(L)6 (1, UiO-67@N).12 The MOF is composed of non-toxic Zr(IV) metal centres and remains highly stable in water. We envisioned that the sizes of the pore windows (11.5 Å and 23 Å), which are larger than the size of the analytes, could permit easy diffusion of analytes inside the MOF, keeping the electron rich MOF and electron deficient nitro analytes in close proximity (Scheme 1). Additionally, the free basic sites (pyridyl) may allow selective interaction between TNP and the MOF, giving rise to an efficient response. The guest free MOF (1 0 ) when dispersed in water exhibited strong fluorescence upon excitation at 320 nm (Fig. S7, ESI†). To trace the explosive sensing ability of 1 0 , changes in the fluorescence intensity of 1 0 dispersed in water towards different nitro aromatic compounds like TNP, TNT, 2,4-DNT, 2,6dinitrotoluene (2,6-DNT), 1,3-dinitrobenzene (DNB), NB and 1,3,5-trinitro-1,3,5-triazacyclohexane (nitro-amine RDX), and nitro-aliphatic compounds such as 2,3-dimethyl-2,3-dinitrobutane (DMNB) and nitromethane (NM), in aqueous solutions were investigated (Fig. 1 and Fig. S8–S15, ESI†). Fig. 1 shows the changes in the fluorescence spectra of 1 0 with increasing amounts of TNP in water. As expected the incremental addition of TNP to 1 0 resulted in fast and high fluorescence quenching (73%). Fluorescence quenching can be clearly observed for TNP concentrations of as low as 2.6 mM. In contrast, all of the other nitro analytes had minor effects on the fluorescence intensity of 1 0 (Fig. 1b). This clearly demonstrates the high selectivity of 1 0 towards TNP over other nitro analytes. From Stern–Volmer (SV) plots in water we were able to calculate the quenching constants and analyse the quenching efficiency of the analytes (Fig. 2) using the SV equation (I0/I) = KSV[A] + 1, where I0 and I are the fluorescence intensities before and after the addition of the respective analyte, [A] is the molar concentration of the analyte and KSV is the quenching constant (M1). At low TNP concentrations a linear increase in the SV plot was observed, which upon further increasing the concentration diverged from linearity and began to bend upwards, while other nitro analytes showed linear increases in the SV plots. This non-linear nature of the SV plot of TNP suggests self-absorption, a combination of static and dynamic quenching or an energy transfer process between TNP and the MOF.11 The fitting of the SV plot for TNP gave a quenching constant of 2.9 104 M1, which is amongst the highest values known for MOFs. Notably, the quenching constant for TNP was found to be comparable to organic polymers
8916 | Chem. Commun., 2014, 50, 8915--8918
Fig. 1 (a) The change in the fluorescence intensity of 10 in water upon incremental addition of aqueous 1 mM TNP solution (lex = 320, [10 ] = 0.5 mg mL1). (b) The fluorescence quenching efficiency for different analytes.
Fig. 2 Stern–Volmer (SV) plots for various nitro analytes in water ([1 0 ] = 0.5 mg mL1).
and is much higher than those of TNT, RDX or other nitro analytes, demonstrating the super-quenching ability of TNP towards 1 0 .13 Encouraged by these results, the selectivity of 1 0 towards TNP in the presence of other nitro analytes in water was investigated. The initial fluorescence spectrum of 1 0 dispersed in water was recorded. To this, aqueous TNT solution (40 mL in two equal portions) was added so that high affinity basic sites were accessible to TNT. Negligible fluorescence quenching was observed upon TNT addition. To this, an equal amount of aqueous TNP (40 mL) was added which resulted in significant quenching (Fig. 3). The trend was also repeated in the subsequent addition cycles and the quenching ability of TNP remained unaffected. Similar results were observed when other
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Fig. 3 The decrease in fluorescence intensities upon the addition of various nitro analytes (1 mM) followed by TNP (1 mM) in aqueous media.
nitro analytes were used instead of TNT. This ascertains the exceptional selectivity of 1 0 towards TNP even in the presence of competing nitro analytes in water. We also checked the interference from the mineral acid HCl on the fluorescence of 1 0 . The addition of HCl showed a negligible effect on the fluorescence intensity of 1 0 (Fig. S16, ESI†). The high selectivity and sensitivity of 1 0 towards TNP in water makes 1 0 a reliable and efficient in-field sensor for TNP that works in aqueous media. Usually, the conduction band (CB) of the electron rich MOF lies higher than the LUMO energies of the nitro analytes and upon excitation the excited electron from the CB transfers to the LUMO orbitals of the nitro analytes, thus quenching the fluorescence intensity.14 The efficient fluorescence quenching observed for TNP is in accordance with the low LUMO energy of TNP compared to the other nitro analytes (Fig. S18, ESI†). However, the correlation between the quenching efficiency and corresponding LUMO energies of the nitro analytes suggests that the electron transfer is not the only mechanism contributing to fluorescence quenching. As mentioned previously the non-linear trend of the SV plot for TNP suggests that a resonance energy transfer mechanism occurs in fluorescence quenching. The greater the spectral overlap between the absorbance spectrum of the analyte and the emission spectrum of the MOF, the higher the probability of energy transfer and hence fluorescence quenching. Fig. 4 shows that the greatest spectral overlap is between TNP and 1 0
Fig. 4 Spectral overlap between normalized absorbance spectra of nitro analytes and the normalized emission spectra of 1 0 in water.
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Communication
while all of the other nitro analytes showed negligible spectral overlap with the emission spectrum of 1 0 . Additionally, the confinement of analytes in the MOF pores keeps the MOF and analytes in close proximity, improving the energy transfer process. Thus it is clear that TNP can efficiently quench the fluorescence of 1 0 via both electron and long range energy transfer processes, which is in contrast to other nitro analytes which quench fluorescence by an electron transfer process only. Additionally, the increasing quenching efficiency with increasing acidity of the phenolic analytes and a red shift in the emission maxima upon the addition of TNP suggested the presence of electrostatic interaction between TNP and the MOF, which is absent for other nitro analytes (Fig. 1a and Fig. S19– S21, ESI†). Thus, due to the occurrence of electron transfer and energy transfer processes in addition to electrostatic interaction, TNP shows highly selective and sensitive fluorescence quenching responses over other nitro analytes in water. In conclusion, a fluorescent porous MOF demonstrates highly selective and sensitive detection of TNP in aqueous media even in the presence of competing nitro analytes. The occurrence of both electron and energy transfer processes, in addition to electrostatic interaction between the MOF and TNP, contribute to the unprecedented selective fluorescence quenching. The present work for the first-time demonstrates the potential of a fluorescent MOF for real-time aqueous phase explosive detection for environmental application. This work was supported by IISER, Pune, DST (GAP/DST/ CHE-12-0083) and DAE (2011/20/37C/06/BRNS) grants. S.S.N. thank CSIR, India for research fellowship. A.V.D. thank IISER, Pune for research fellowship. We acknowledge HEMRL, Pune for TNT and RDX samples.
Notes and references 1 (a) Y. Salinas, R. Martinez-Manez, M. D. Marcos, F. Sancenon, A. M. Castero, M. Parra and S. Gil, Chem. Soc. Rev., 2012, 41, 1261; (b) S. W. Thomas III, G. D. Joly and T. M. Swager, Chem. Soc. Rev., 2007, 36, 1339; (c) M. E. Germain and M. J. Knapp, Chem. Soc. Rev., 2009, 38, 2543. 2 G. He, H. Peng, T. Liu, M. Yang, Y. Zhang and Y. Fang, J. Mater. Chem., 2009, 19, 7347. 3 (a) P. G. Thorne and T. F. Jenkins, Field Anal. Chem. Technol., 1997, 1, 165; (b) K. M. Wollin and H. H. Dieter, Arch. Environ. Contam. Toxicol., 2005, 49, 18. 4 B. Xu, X. Wu, H. Li, H. Tong and L. Wang, Macromolecules, 2011, 44, 5089. 5 H. Sohn, M. J. Sailor, D. Magde and W. C. Trogler, J. Am. Chem. Soc., 2003, 125, 3821. 6 Y. Xin, Q. Wang, T. Liu, L. Wang, J. Li and Y. Fang, Lab Chip, 2012, 12, 4821. 7 (a) S. Richardson, H. S. Barcena, G. A. Turnbull, P. L. Burn and I. D. W. Samuel, Appl. Phys. Lett., 2009, 95, 063305; (b) D. Li, J. Liu, R. T. K. Kwok, Z. Liang, B. Z. Tang and J. Yu, Chem. Commun., 2012, 48, 7167; (c) M. S. Meaney and V. L. McGuffin, Anal. Bioanal. Chem., 2008, 391, 2557; (d) E. S. Snow, F. K. Perkins, E. J. Houser, S. C. Badescu and T. L. Reinecke, Science, 2005, 307, 1942; (e) B. Gole, S. Shanmugaraju, A. K. Bar and P. S. Mukherjee, Chem. Commun., 2011, 47, 10046; ( f ) S. Shanmugaraju, S. A. Joshi and P. S. Mukherjee, Inorg. Chem., 2011, 50, 11736; ( g) K. K. Kartha, S. S. Babu, S. Srinivasan and A. Ajayaghosh, J. Am. Chem. Soc., 2012, 134, 4834. 8 S.-R. Zhang, D.-Y. Du, J.-S. Qin, S.-J. Bao, S.-L. Li, W.-W. He, Y.-Q. Lan, P. Shen and Z.-M. Su, Chem. – Eur. J., 2014, 20, 3589.
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Communication 9 (a) H. Furukawa, K. E. Cordova, M. Okeeffe and O. M. Yaghi, Science, 2013, 341, 1230444; (b) S. S. Nagarkar, A. V. Desai and S. K. Ghosh, Chem. – Asian J., 2014, DOI: 10.1002/asia.201402004; (c) L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. V. Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105; (d) P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurine, P. Couvreur, G. Ferey, R. Morris and C. Serre, Chem. Rev., 2012, 112, 1232; (e) S.-L. Li and Q. Xu, Energy Environ. Sci., 2013, 6, 1656. 10 (a) Z. Hu, B. J. Deibert and J. Li, Chem. Soc. Rev., 2014, DOI: 10.1039/ c4cs00010b; (b) Y. Cui, Y. Yue, G. Qian and B. Chen, Chem. Rev., 2012, 112, 1126; (c) A. Lan, K. Li, H. Wu, D. H. Olson, T. J. Emge, W. Ki, M. Hong and J. Li, Angew. Chem., 2009, 121, 2370; (d) A. Lan, K. Li, H. Wu, D. H. Olson, T. J. Emge, W. Ki, M. Hong and J. Li, Angew. Chem., Int. Ed., 2009, 48, 2334; (e) S. Das and P. K. Bharadwaj, Inorg. Chem., 2006, 45, 5257; ( f ) C. Wang and W. Lin, J. Am. Chem.
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11 12 13 14
Soc., 2011, 133, 4232; ( g) C. Zhang, Y. Che, Z. Zhang, X. Yang and L. Zang, Chem. Commun., 2011, 47, 2336; (h) Z. Zhang, S. Xiang, X. Rao, Q. Zheng, F. R. Fronczek, G. Qian and B. Chen, Chem. Commun., 2010, 46, 7205; (i) H. L. Jiang, Y. Tatsu, Z. H. Lu and Q. Xu, J. Am. Chem. Soc., 2010, 132, 5586; ( j) H. Xu, F. Liu, Y. Cui, B. Chen and G. Qian, Chem. Commun., 2011, 47, 3153; (k) B. Gole, A. K. Bar and P. S. Mukherjee, Chem. – Eur. J., 2014, 20, 2276; (l ) G.-Y. Wang, C. Song, D.-M. Kong, W.-J. Ruan, Z. Chang and Y. Li, J. Mater. Chem. A, 2014, 2, 2213. S. S. Nagarkar, B. Joarder, A. K. Chaudhari, S. Mukherjee and S. K. Ghosh, Angew. Chem., Int. Ed., 2013, 52, 2881. P. V. Dau, M. Min and S. M. Cohen, Chem. Sci., 2013, 4, 601. J. C. Sanchez, A. G. DiPasquale, A. L. Rheingold and W. C. Trogler, Chem. Mater., 2007, 19, 6459. S. J. Toal and W. C. Trogler, J. Mater. Chem., 2006, 16, 2871.
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COMMUNICATION
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Cite this: Chem. Commun., 2014, 50, 8915
A fluorescent metal–organic framework for highly selective detection of nitro explosives in the aqueous phase†
Received 24th April 2014, Accepted 16th May 2014
Sanjog S. Nagarkar, Aamod V. Desai and Sujit K. Ghosh*
DOI: 10.1039/c4cc03053b www.rsc.org/chemcomm
Selective and sensitive nitro explosive detection by a porous luminescent metal–organic framework has been reported. For the first time MOF based selective explosive detection in the presence of other nitro analytes in aqueous media is demonstrated.
Selective and sensitive detection of highly explosive and explosivelike substances has become a serious issue concerning national security and environmental protection.1 Nitro explosives such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), 2,4,6trinitrophenol (TNP), and nitrobenzene (NB) are the main constituents of numerous unexploded land mines used during World War II, and even in land mines today.1c Amongst these nitro explosives TNP has higher explosive power than TNT and is commonly used in dyes, fireworks, matches, glass, and leather industries.2 In addition to its explosive nature, TNP has also been recognized as a toxic pollutant. TNP and its mammalian metabolite, picramic acid, are known for their mutagenic properties.3 TNP is released into the environment during commercial production and use, leading to the contamination of soil and aquatic systems. Thus the selective and sensitive detection of TNP present in soil and ground water is very important for tracing buried explosives and environmental monitoring near industrial areas. However, selective and sensitive detection of TNP in the presence of other nitro compounds in water is a challenge due to their strong electron affinities, leading to false responses.4 Although current explosive detection methods, including trained canines and modern analytical techniques, are selective and accurate they suffer from disadvantages like high operational cost and portability issues during in-field use.5 Fluorescence based detection methods have attracted great attention recently by virtue of their high sensitivities, portability, short response times, and applicability in both the solid and solution Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pashan, Pune, India 411008. E-mail: [email protected]; Fax: +91 20 2589 8022; Tel: +91 20 2590 8076 † Electronic supplementary information (ESI) available: For details of MOF synthesis, PXRD patterns and photo physical studies of the MOF. See DOI: 10.1039/c4cc03053b
This journal is © The Royal Society of Chemistry 2014
phase.6 A variety of materials including conjugated organic molecules, nanoparticles and metal complexes have been employed for fluorescence based explosive detection.7 Despite this fact, their widespread use is limited due to stability, toxicity, multi-step processing and lack of control over molecular organization.8 Metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) have been extensively used for gas storage/separation, catalysis, sensing, optoelectronics, clean energy and biomedical applications owing to their high surface areas, designable architectures and host–guest interactions.9 In particular, as luminescent sensors MOFs provide several advantages over conventional fluorophores.10a,b Their designable architectures allow improved host– guest interactions and act as pre-concentrators for target analytes. Additionally, the immobilization of organic struts in MOFs gives rise to strong emissions due to reduced non-radiative relaxation. The infinite number of choices of organic linkers and/or metal centers allow the fine tuning of the electronic properties of MOFs. Furthermore, the introduction of secondary functional groups endorses preferred binding of the chosen analyte giving rise to better selectivity. For in-field selective detection of nitro explosives present in soil and ground water, probe working in aqueous media is highly desirable. Although MOFs have been employed for liquid phase explosive detection, to the best of our knowledge there are no reports of MOF based probes which can selectively detect nitro explosives in aqueous media.10 Recently, we reported the fluorescent MOF [Cd(NDC)0.5(PCA)]Gx, which exhibits a highly selective response towards TNP even in the presence of competing nitro analytes due to free basic sites.11 However, like most MOFs its poor water stability limits its application to organic solvents. The pore size, which is smaller than the analytes, prevents the pre-concentration of analytes in the coordination space of the MOF. Moreover, the toxic nature of Cd(II) hinders its widespread use in environmental applications. In our efforts to develop fluorescent MOFs that work in aqueous media for the selective detection of nitro explosives, we became interested in 2-phenylpyridine-5,4 0 -dicarboxylic acid (LH2) and the Zr(IV)
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Scheme 1 A fluorescent MOF based sensor for highly selective nitro explosive detection in the aqueous phase.
based MOF Zr6O4(OH)4(L)6 (1, UiO-67@N).12 The MOF is composed of non-toxic Zr(IV) metal centres and remains highly stable in water. We envisioned that the sizes of the pore windows (11.5 Å and 23 Å), which are larger than the size of the analytes, could permit easy diffusion of analytes inside the MOF, keeping the electron rich MOF and electron deficient nitro analytes in close proximity (Scheme 1). Additionally, the free basic sites (pyridyl) may allow selective interaction between TNP and the MOF, giving rise to an efficient response. The guest free MOF (1 0 ) when dispersed in water exhibited strong fluorescence upon excitation at 320 nm (Fig. S7, ESI†). To trace the explosive sensing ability of 1 0 , changes in the fluorescence intensity of 1 0 dispersed in water towards different nitro aromatic compounds like TNP, TNT, 2,4-DNT, 2,6dinitrotoluene (2,6-DNT), 1,3-dinitrobenzene (DNB), NB and 1,3,5-trinitro-1,3,5-triazacyclohexane (nitro-amine RDX), and nitro-aliphatic compounds such as 2,3-dimethyl-2,3-dinitrobutane (DMNB) and nitromethane (NM), in aqueous solutions were investigated (Fig. 1 and Fig. S8–S15, ESI†). Fig. 1 shows the changes in the fluorescence spectra of 1 0 with increasing amounts of TNP in water. As expected the incremental addition of TNP to 1 0 resulted in fast and high fluorescence quenching (73%). Fluorescence quenching can be clearly observed for TNP concentrations of as low as 2.6 mM. In contrast, all of the other nitro analytes had minor effects on the fluorescence intensity of 1 0 (Fig. 1b). This clearly demonstrates the high selectivity of 1 0 towards TNP over other nitro analytes. From Stern–Volmer (SV) plots in water we were able to calculate the quenching constants and analyse the quenching efficiency of the analytes (Fig. 2) using the SV equation (I0/I) = KSV[A] + 1, where I0 and I are the fluorescence intensities before and after the addition of the respective analyte, [A] is the molar concentration of the analyte and KSV is the quenching constant (M1). At low TNP concentrations a linear increase in the SV plot was observed, which upon further increasing the concentration diverged from linearity and began to bend upwards, while other nitro analytes showed linear increases in the SV plots. This non-linear nature of the SV plot of TNP suggests self-absorption, a combination of static and dynamic quenching or an energy transfer process between TNP and the MOF.11 The fitting of the SV plot for TNP gave a quenching constant of 2.9 104 M1, which is amongst the highest values known for MOFs. Notably, the quenching constant for TNP was found to be comparable to organic polymers
8916 | Chem. Commun., 2014, 50, 8915--8918
Fig. 1 (a) The change in the fluorescence intensity of 10 in water upon incremental addition of aqueous 1 mM TNP solution (lex = 320, [10 ] = 0.5 mg mL1). (b) The fluorescence quenching efficiency for different analytes.
Fig. 2 Stern–Volmer (SV) plots for various nitro analytes in water ([1 0 ] = 0.5 mg mL1).
and is much higher than those of TNT, RDX or other nitro analytes, demonstrating the super-quenching ability of TNP towards 1 0 .13 Encouraged by these results, the selectivity of 1 0 towards TNP in the presence of other nitro analytes in water was investigated. The initial fluorescence spectrum of 1 0 dispersed in water was recorded. To this, aqueous TNT solution (40 mL in two equal portions) was added so that high affinity basic sites were accessible to TNT. Negligible fluorescence quenching was observed upon TNT addition. To this, an equal amount of aqueous TNP (40 mL) was added which resulted in significant quenching (Fig. 3). The trend was also repeated in the subsequent addition cycles and the quenching ability of TNP remained unaffected. Similar results were observed when other
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Fig. 3 The decrease in fluorescence intensities upon the addition of various nitro analytes (1 mM) followed by TNP (1 mM) in aqueous media.
nitro analytes were used instead of TNT. This ascertains the exceptional selectivity of 1 0 towards TNP even in the presence of competing nitro analytes in water. We also checked the interference from the mineral acid HCl on the fluorescence of 1 0 . The addition of HCl showed a negligible effect on the fluorescence intensity of 1 0 (Fig. S16, ESI†). The high selectivity and sensitivity of 1 0 towards TNP in water makes 1 0 a reliable and efficient in-field sensor for TNP that works in aqueous media. Usually, the conduction band (CB) of the electron rich MOF lies higher than the LUMO energies of the nitro analytes and upon excitation the excited electron from the CB transfers to the LUMO orbitals of the nitro analytes, thus quenching the fluorescence intensity.14 The efficient fluorescence quenching observed for TNP is in accordance with the low LUMO energy of TNP compared to the other nitro analytes (Fig. S18, ESI†). However, the correlation between the quenching efficiency and corresponding LUMO energies of the nitro analytes suggests that the electron transfer is not the only mechanism contributing to fluorescence quenching. As mentioned previously the non-linear trend of the SV plot for TNP suggests that a resonance energy transfer mechanism occurs in fluorescence quenching. The greater the spectral overlap between the absorbance spectrum of the analyte and the emission spectrum of the MOF, the higher the probability of energy transfer and hence fluorescence quenching. Fig. 4 shows that the greatest spectral overlap is between TNP and 1 0
Fig. 4 Spectral overlap between normalized absorbance spectra of nitro analytes and the normalized emission spectra of 1 0 in water.
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Communication
while all of the other nitro analytes showed negligible spectral overlap with the emission spectrum of 1 0 . Additionally, the confinement of analytes in the MOF pores keeps the MOF and analytes in close proximity, improving the energy transfer process. Thus it is clear that TNP can efficiently quench the fluorescence of 1 0 via both electron and long range energy transfer processes, which is in contrast to other nitro analytes which quench fluorescence by an electron transfer process only. Additionally, the increasing quenching efficiency with increasing acidity of the phenolic analytes and a red shift in the emission maxima upon the addition of TNP suggested the presence of electrostatic interaction between TNP and the MOF, which is absent for other nitro analytes (Fig. 1a and Fig. S19– S21, ESI†). Thus, due to the occurrence of electron transfer and energy transfer processes in addition to electrostatic interaction, TNP shows highly selective and sensitive fluorescence quenching responses over other nitro analytes in water. In conclusion, a fluorescent porous MOF demonstrates highly selective and sensitive detection of TNP in aqueous media even in the presence of competing nitro analytes. The occurrence of both electron and energy transfer processes, in addition to electrostatic interaction between the MOF and TNP, contribute to the unprecedented selective fluorescence quenching. The present work for the first-time demonstrates the potential of a fluorescent MOF for real-time aqueous phase explosive detection for environmental application. This work was supported by IISER, Pune, DST (GAP/DST/ CHE-12-0083) and DAE (2011/20/37C/06/BRNS) grants. S.S.N. thank CSIR, India for research fellowship. A.V.D. thank IISER, Pune for research fellowship. We acknowledge HEMRL, Pune for TNT and RDX samples.
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